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I believe it outlined the reason at the end of the video

does prism act like a lendiing/marging ?

It provides you the ability to create a portfolio of different cryptocurrency. Its not exactly lending.

but how you can get different crypto if you only use ethereum?

Let say you have $1000 in ETH. You'll invest a percentage of each currency, for example 1/3 Dash, 1/3 Monero, 1/3 Litecoin etc.

ah ok
and whe you withdraw it give you only ethereum minus the fee ?

Exactly! You'll receive any profits less the fees.